Recommendations of Backer for Halogen Contact Time in Water Based on Temperature and Halogen Concentration

Water Temperature	Halogen 2 ppm	Halogen 4 ppm	Halogen 8 ppm
5°C	240 minutes	180 minutes	60 minutes
15°C	180 minutes	60 minutes	30 minutes
25°C	60 minutes	45 minutes	15 minutes

 

These times are higher than usual recommendations to take into account any uncertainty about residual halogen and the time to kill Giardia cysts in very cold water.

 

If the data at 5°C and 25°C is analyzed, it can be seen that the contact time at 4 ppm is 3 times the time at 8 ppm, and the contact time at 2 ppm is 4 times the concentration at 8 ppm. This relationship is not seen with the 15°C data.

 

If the data at 8 ppm is analyzed then time at 15°C is twice as much and at 5° is four times as much.

 

All of this suggests 2 alternatives to the 15°C contact times to maintain these relationships:

 

Halogen	Original	Option 1	Option 2
2 ppm	180 minutes	120 minutes	180 minutes
4 ppm	60 minutes	90 minutes	135 minutes
8 ppm	30 minutes	30 minutes	45 minutes

 

If option 1 is combined with the data at 5°C and 25°, then

 

contact time at 5°C =

= 300 - 30 ppm

 

contact time at 15°C =

= 150 - 15 ppm

 

contact time at 25°C =

= 75 - 7.5 ppm

 

general contact equation =

= (10 * X) - (X * ppm)

 

If X is mapped vs temperature:

 

X = (0.03786 * ((temperature)^2)) - (2.251 * (temperature)) + 40.20

 

Combining these equations allow calculations of halogen contact times for water temperatures from 5 to 25°C and halogen concentrations of 2 to 8 ppm.